There are two classifications of flow measurement in hydraulic turbines: absolute and relative. Absolute is divided into volumetric flow rate and weight flow rate. The terms apply to both reaction and impulse turbines. A reaction turbine is classified as such because of a pressure change across a turbine runner. Impulse turbines have no reaction, i.e., no change in pressure.
Relative flow measure means that flow is measured in relative terms. A surrogate parameter is measured and the relative flow is calculated (implied) from that measured surrogate. The actual flow rate is estimated or “indexed” via the flow's effect on the surrogate, thus a “relative efficiency test” is termed an index test. For example, as the flow in a turbine increases the water level in the gate slot decreases due to increased velocity head in the passage under the slot and increased trash rack losses. Thus, as flow increases the difference between the gate slot water height and the forebay height increases. Thus, the difference in height may be used as a surrogate measure of flow.
A standard method of measuring relative flow employs Winter-Kennedy piezometers or WK's. Two WK's are placed on the spiral (or semi-spiral) case of a turbine, one on the inside and the other on the outside in the same radial plane. The difference in angular momentum due to different radial displacement from the center of rotation generates a difference in piezometric pressure on the two taps. Since angular momentum is a function of the square of the velocity, the relative flow rate is proportional to the square root of the difference in piezometric pressure. Since this method measures change in angular momentum, it is measuring a relative weight flow rate. However, when it is independently calibrated, it is done in terms of an absolute volumetric flow rate.
Relative flow is often used to establish a relative efficiency profile, e.g., the power level at which peak efficiency occurs and the “one percent” operating limits, since a 1% change in relative efficiency equates to a 1% change in absolute efficiency. Further, in Kaplan turbines, relative flow may be used to establish the optimum blade to gate cam curve.
A relative flow measure applies only to the unit on which it is measured, i.e., the same absolute flow rate in two different units will produce different calibrations of the WK's. Thus, the relative efficiency of two units can not be compared and “absolute efficiency” measures must be employed for any meaningful comparison.
Techniques exist to measure absolute flow in hydraulic turbines, but most have limited application, such as requiring a constant cross section over a length of the flow passage. Thus, for run of the river Kaplan turbines where the shape of the water in the passage is continually changing, only a few methods of questionable accuracy are available.
The current ASME test code, PTC 19-2002, describes the following methods of flow measuring: current meter, pressure-time, ultrasonic, Venturi meter, dye solution, and volumetric as well as the thermodynamic method of measuring efficiency. The 1992 ASME test code described other methods to include pitometer, pitot tube, and salt velocity. Other methods include: traveling screen, weir and scintillation.
Conventional current meters used today are of the type employing the “point velocity methods.” Point velocity methods measure point velocities over a cross section and integrate the resulting measurements to yield mean velocity and from that, flow rate. Current meters employ small propellers mounted to a frame in a water passage. The speed of rotation of the propeller is proportional to the current velocity. These meters are calibrated in a laboratory flume. Variance in current meter measurements is introduced by the obstruction to flow from the frame itself and the misalignment of the meter with respect to the velocity vector.
Another method is the pressure-time method, or Gibson method, that measures a transient pressure increase or “water hammer” resultant from a rapid closure of the wicket gates causing the momentum of the fluid column in the penstock to decrease. Integrating the change in momentum over time yields a weight flow rate that must be converted to a volumetric flow rate.
There are three ultrasonic methods for measuring flow. The first measures separate transit times of pulses sent obliquely upstream and downstream and averages the velocity vectors of the fluid crossing the path taken by the ultrasonic pulses. Other methods measure the refraction of an ultrasonic beam by fluid velocity or by measuring the Doppler frequency shift of an ultrasonic signal reflected by flowing water or by moving particles. The first method is accepted by the ASME code and its accuracy is a function of the number of acoustic paths averaged over the monitored cross section.
Venturi meters are used to measure absolute flow in piping such as that used in smaller penstocks of hydraulic turbines or in the lab. A Venturi, also termed a DeLaval nozzle, measures the difference between the pressure head at the inlet and at the minimum cross section to establish flow rate.
The dye dilution method injects a dye tracer upstream at a constant rate into the flow. Samples are drawn downstream upon complete mixing and analyzed to determine concentration. Flow rate is proportional to the dilution experienced by the dye.
The volumetric method establishes average flow rate by monitoring the change in a reservoir's fluid height over time. The method requires a survey of the reservoir to establish an accurate relationship between volume change and fluid height.
The thermodynamic method measures the efficiency of the turbine directly by accurately measuring the temperature of the water before and after the turbine and calculates the flow rate from the difference in the temperature. Because of the high specific heat of water, this method is restricted to turbines having a hydraulic head greater than 300 feet.
The Cole Reversible Pitometer is a point source method that uses the difference in pressure created by flow over a pair of movable orifices to yield local velocity. One orifice faces directly upstream and the other downstream. The upstream orifice measures the water flow and its velocity directly and the downstream orifice measures the effect induced by the suction from the wake of the water flow around the orifice.
The Pitot Static Tube is another point source method like the Cole unit except that the second orifice is at 90° to the upstream facing one, generally on the side of the penetrating pipe used to house the first orifice.
The Allen Salt Velocity method uses a “slug” of salt to increase the electrical conductivity of the water. Knowing where the slug is inserted and where the electrical conductivity of the water containing the dispersed slug is taken downstream, the time difference is used to establish average velocity and then calculate average flow.
The Anderson Traveling Screen method employs an impermeable screen mounted on tracks that is inserted perpendicular to the flow. It is very accurate but suitable for measuring flow only in relatively narrow open channels of uniform cross section due to its configuration.
Weirs are used to measure flow in an open channel, being analogous to a dam being overtopped. Sharp crested weirs have a thin knife-edge top whereas a broad-crested weir allows the water flowing on top to reside there for a sufficient time to establish a critical depth. In both types, the height of the water column behind the weir is measured to estimate flow rate.
The scintillation method employs acoustic signals to map the passage of a turbulence pattern on a horizontal line in a vertical cross section and records the passage of that same “map” as it occurs a short distance downstream before momentum can change the pattern. The time to travel the short distance yields an average local velocity. Integrating the local velocities across a series of acoustic beams in a vertical cross section then yields a flow rate estimate.
For all methods of measuring absolute flow, only a few may be used with the large Kaplan turbines. The pressure-time (Gibson) and Allen (salt) methods are eliminated because of the absence of a constant flow cross section over a significant length. The Venturi method is used only with penstocks. The low hydraulic head of a Kaplan turbine eliminates the Thermodynamic Method. The volumetric and Anderson (moving screen) methods can not be used where the natural river is both the forebay and the tailrace. Kaplan turbines on rivers have multiple intake bays and each bay on a turbine has different flow rates, eliminating the dye dilution method that requires an injection proportional to flow rate in each bay. The pitometer methods are not useful for large intakes used in Kaplan turbines since the number of pitometer lines needed to be drawn up the gate or bulkhead slots would be unmanageable.
Ultrasonic methods have been used on Kaplan turbines with some success, however, the large intake barrels require a large number of transducers that must be permanently mounted to be aligned obliquely to the flow, thus the initial setup cost per unit is high.
The scintillation method may be applied to “run of the river” Kaplan turbines, however, experience has shown that the method significantly underestimates actual flow. Further, the cost of a scintillation frame is about $300,000 in 2007 dollars for a typical intake gate slot, while the test contractor charges about $75,000-100,000 per test. Further, the flow estimates are not available until several months after test.
Currently there are devices commercially available that can measure flow in turbine units served by penstocks. Employing these devices is so expensive that many projects choose not to use them. For turbines that have short intakes without penstocks, there is no reasonably cost effective and accurate method for measuring absolute flow. By accurately measuring absolute hydraulic flow (Q), the maximum efficiency for individual turbine units may be determined. For example, accurately measuring Q identifies those units that are most efficient and should be employed the most, i.e., economic unit dispatch.
Thus, what is needed is a cost effective method employing commercial off-the-shelf (COTS) components that may be employed in various configurations without unduly interfering with operations of existing systems. The measurement apparatus should also provide “real time” accurate absolute flow measurements. Select embodiments of the present invention provide this capability.